Rubber composition for inner liner

ABSTRACT

The rubber composition of the present invention is used for producing an inner liner of a pneumatic tire, and is obtained by compounding a rubber component and a layered or plate-like mineral. The layered or plate-like mineral has an aspect ratio of 3 or more and less than 30. The present invention further provides a production method of the rubber composition and a pneumatic tire which is produced by using the rubber composition. The present invention is applicable to a tire of passenger vehicles, bus or truck, and a tire of airplane. According to the present invention, the air permeation resistance is remarkably improved and the workability is also improved. Therefore, the rapture and hole defect of a non-vulcanized sheet during the tire construction can be avoided. By using the rubber composition, the gauge of the inner liner of pneumatic tire can be reduced while maintaining the inner pressure of tire, thereby reducing the weight of tire.

TECHNICAL FIELD

The present invention relates to a rubber composition for inner liner ofa pneumatic tire. More specifically, the present invention relates to arubber composition which provides an inner liner having an excellent airpermeation resistance and an improved workability because of itsspecific compounding formulation containing a butyl rubber, ahalogenated butyl rubber or a diene rubber, and to a pneumatic tirehaving its weight reduced by using the rubber composition.

BACKGROUND ART

To prevent air leakage and maintain the air pressure of a tire constant,an inner liner made of a less air-permeability rubber such as butylrubber and a halogenated butyl rubber is generally provided on the innerwall of a pneumatic tire. However, since the strength of anon-vulcanized rubber is reduced with increasing content of butylrubber, a non-vulcanized rubber sheet is susceptible to rapture and holedefect. Particularly, when an inner liner is of smaller gauge, a cordprovided inside of the tire easily comes out of the inner liner duringthe tire construction.

An airplane tire is sometimes exposed to an atmosphere as low as −65° C.Also, a heavy weight vehicle such as truck and bus is occasionallyparked in extremely cold region, thereby exposing its tire to a lowtemperature as low as −50° C. Therefore, if the inner liner of a tire ofa heavy weight vehicle is composed mainly of butyl rubber or ahalogenated butyl rubber, each having a high glass transitiontemperature, cracking is liable to occur in the inner liner.

To meet the recent social demand for saving energy, various methods havebeen proposed in reducing the gauge of inner liner to reduce the weightof a tire. For example, Japanese Patent Application Laid-Open Nos.7-40702 and 7-81306 propose to use a nylon film layer or apoly(vinylidene chloride) layer in place of butyl rubber. JapanesePatent Application Laid-Open No. 10-26407 proposes to use a film of acomposition comprising a thermoplastic resin such as polyamide resinsand polyester resins blended with an elastomer.

The proposed methods using the above films are, in some degree,successful in reducing the weight of tire. However, since the matrix ofeach film is a crystalline resin, the crack resistance and the flexuralfatigue resistance are poor, particularly, when used at temperatureslower than 5° C., as compared with a layer generally used, which is madeof a composition compounded with butyl rubber. In addition, the use of amatrix crystalline resin makes the process for producing a tirecomplicated.

Also known are a method of compounding flat mica as a filler for arubber composition (Japanese Patent Application Laid-Open No. 11-140234)and a method of compounding a clay (Japanese Patent ApplicationLaid-Open No. 5-017641). In these methods, however, the filler is notuniformly dispersed during a rubber kneading when the filling amount isincreased, thereby likely to reduce the flexural fatigue resistance andthe low-temperature durability because of insufficient dispersion.

DISCLOSURE OF INVENTION

In view of the above state of prior art, an object of the presentinvention is to provide a rubber composition for inner liner which isexcellent in the air permeation resistance and improved in theworkability before vulcanization. Another object of the presentinvention is to provide a pneumatic tire which has an inner liner withits gauge being made extremely thinner by the use of the rubbercomposition while maintaining the ability of retaining the inner tirepressure, and shows a good crack resistance (low-temperature durability)and a flexural fatigue resistance during the use under low-temperatureconditions.

As a result of extensive study for developing a rubber compositionhaving the above superior properties, the inventors have found that theabove objects are achieved by using a specific rubber compositioncompounded with a layered or plate-like mineral. The present inventionhas been accomplished on the basis of this finding.

Thus, the present invention provides a rubber composition for innerliner of pneumatic tire, which comprises a rubber component and alayered or plate-like mineral compounded with the rubber component, thelayered or plate-like mineral having an aspect ratio of 3 or more andless than 30.

In the rubber composition, the rubber component preferably comprises 40to 100% by weight of at least one rubber selected from the groupconsisting of butyl rubber and a halogenated butyl rubber and 60% byweight or less of a diene rubber, and a weight-basis compounding amountA of the layered or plate-like mineral per 100 parts by weight of therubber component preferably satisfies the following Formula I:1<A×D<200  (I)wherein D represents a thickness (mm) of an inner liner rubber layer.

The present invention further provides a rubber composition for innerliner which is applied to an airplane tire or a tire for heavy weightvehicles used in an extremely low temperature region, in which therubber component preferably comprises 60 to 100% by weight of a dienerubber.

The present invention still further provides a pneumatic tire having aninner liner made of the above rubber composition.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration for explaining the aspect ratio;

FIG. 2 is a schematic illustration for showing the layered or plate-likeminerals contained in the inner liner of the present invention; and

FIG. 3 is a cross-sectional view showing a left half of the pneumatictire of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the rubber composition for inner liner of the present invention, alayered or plate-like mineral having an aspect ratio of 3 or more andless than 30 is compounded with a rubber component.

As the rubber component of the rubber composition of the presentinvention, either of a butyl-based rubber or a diene rubber may be used.The butyl-base rubber preferably contains a halogenated butyl rubbersuch as chlorinated butyl rubbers, brominated butyl rubbers and modifiedproducts thereof. For example, “Enjay Butyl HT10-66” (trade mark,manufactured by Enjay Chemical Co., Inc.) is available for thechlorinated butyl rubber, and “Bromobutyl 2255” (trade mark,manufactured by Exxon Company) is available for the brominated butylrubber. As the modified rubber, usable is a chlorinated or brominatedproduct of a copolymer of isomonoolefin and p-methylstyrene, which isavailable as “Expro 50” (trade mark, manufactured by Exxon). The dienerubber to be blended with such a halogenated butyl rubber is, forexample, natural rubber, synthetic isoprene rubber (IR),poly(cis-1,4-butadiene) (BR), syndiotactic poly(1,2-butadiene) (1,2BR),styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR) orchloroprene rubber (CR). These diene rubbers may be used alone or incombination of two or more.

In view of the air permeation resistance, the rubber componentpreferably comprises 40 to 100% by weight of the butyl-based rubber and60% by weight or less of the diene rubber. The rubber compositioncomprising such a rubber component is suitable for a tire of motorcycle,passenger vehicle, truck, and bus.

For use as a plane tire or a tire of truck or bus, which is used underextremely low temperature conditions, a rubber component comprising 60%by weight or more of the diene rubber is preferred, and a rubbercomponent comprising mainly a natural rubber is more preferred.

The layered or plate-like mineral used in the present invention may beeither natural mineral or synthetic mineral, and not specificallylimited as long as the aspect ratio thereof is 3 or more and less than30. Examples thereof include kaolin, clay, mica, feldspar, hydratedsilica-alumina composite, with a kaolinic clay and mica being preferredand the kaolinic clay being more preferred. The particle size of thelayered or plate-like mineral is preferably 0.2 to 2 μm, and the aspectratio is preferably 5 or more and less than 30, more preferably 8 to 20.When the aspect ratio is 3 or more, a sufficient improvement in the airpermeation resistance is obtained. By limiting the aspect ratio to lessthan 30, the workability can be prevented from being deteriorated. Inthe present invention, the aspect ratio means a ratio of a/b, wherein ais an average major diameter and b is an average minor diameter of 50mineral particles arbitrarily selected, determined by microscopicobservation. In FIG. 1, the major diameter and the minor diameter of aunit particle are schematically illustrated.

The weight-basis compounding amount A of the layered or plate-likemineral per 100 parts by weight of the rubber component preferablysatisfies the following Formula I:1<A×D<200  (I)wherein D represents a thickness (mm) of an inner liner rubber layer. Byregulating the compounding amount within the above range, an excellentimprovement in the air permeation resistance is obtained.

The compounding amount of the layered or plate-like mineral ispreferably 10 to 200 parts by weight, more preferably 20 to 160 parts byweight based on 100 parts by weight of the rubber component. And theamount of carbon black to be used in the rubber composition is 0 to 60parts by weight, preferably 0 to 40 parts by weight, and more preferably5 to 35 parts by weight based on 100 parts by weight of the rubbercomponent.

The total amount of the layered or plate-like mineral and the carbonblack is preferably 50 parts by weight or more. In particular, inconsidering the air permeation resistance, flexural fatigue resistanceand workability, the total amount is preferably 60 to 220 parts byweight.

The type of the carbon black is not specifically limited and may besuitably selected from those conventionally used as the reinforcingfiller of rubber compositions, such as FEF, SRF, HAF, ISAF and SAF. Ofthese carbon blacks, preferred are those having a specific surface areadetermined by nitrogen adsorption (N₂SA) of 26 m²/g to 170 m²/g whenmeasured according to ASTM D3037-88. The carbon black is preferred tohave an iodine adsorption (IA) of 40 mg/g or less when measuredaccording to ASTM D 1510-95 and a dibutyl phthalate adsorption of 100ml/100 g or less when measured by ASTM D2414-97.

As an example of a preferred rubber composition, mention may be made ofa rubber composition for use in inner liner comprises 100 parts byweight of the rubber component comprising at least one butyl-basedrubber selected from butyl rubber and a halogenated rubber, 10 to 50parts by weight of clay, and 10 to 60 parts by weight of the carbonblack, with the total of clay and the carbon black being 50 parts byweight or more.

The rubber composition of the present invention may be furthercompounded with a softening agent such as a naphtene based oil, aparaffin based oil, an aromatic oil and a blown asphalt oil. Thecompounding amount is not specifically limited and suitably selecteddepending on the applications. For example, when the total amount of thecarbon black and the layered or plate-like mineral is relatively small(up to about 100 parts by weight per 100 parts by weight of the rubbercomponent), the softening agent may be compounded one part by weight ormore, preferably 3 to 20 parts by weight per 100 parts by weight of therubber component. The % C_(N) of the naphtene base oil is 30 or more,and the % C_(P) of the paraffin base oil is 60 or more when determinedby a ring analysis (n-d-M method).

To enhance the dispersibility of the layered or plate-like mineralthroughout the rubber composition, a dispersion improver such as silanecoupling agents, dimethylstearylamine and trimethanolamine may be added,if desired, in an amount of 0.1 to 5 parts by weight per 100 parts byweight of the rubber component.

Further, the rubber composition of the present invention may becompounded with organic short fibers made of an organic polymer resin.By compounding organic short fibers, the underlying cords areeffectively prevented from being bared to the surface of an inner linerduring the construction of tire having an inner liner with a smallthickness. The organic short fiber is preferred to have an averagediameter of 1 to 100 μm and an average length of about 0.1 to 0.5 mm.The organic short fiber may be compounded as a composite prepared bykneading the organic short fiber and a non-vulcanized rubber component(hereinafter referred to as “FRR”).

The compounding amount of the organic short fiber is preferably 0.3 to15 parts by weight per 100 parts by weight of the rubber component. Bycompounding 0.3 part by weight or more, the exposure of the underlyingcords to the surface of an inner liner is effectively prevented. Bylimiting the compounding amount to 5 parts by weight or less, theworkability is not adversely affected. The materials of the organicshort fiber may include, but not specifically limited thereto, apolyamide such as nylon 6 and nylon 66, a syndiotacticpoly(1,2-butadiene), an isotactic polypropylene and polyethylene withthe polyamide being preferred.

When the organic short fiber is compounded, an rubber-to-fiber adhesionimprover such as hexamethylenetetramine and resorcinol may be furthercompounded to increase the modulus of a resultant rubber composition.

In addition to the above compounding ingredients, the rubber compositionof the present invention may be further compounded with another additiveused in the rubber art such as vulcanization agents, vulcanizationaccelerators, antioxidants, scorch retarders, zinc white and stearicacid in an amount not adversely affect the effect of the presentinvention.

The rubber composition of the present invention may be produced by aknown method, namely, by kneading in a kneader the rubber component, thelayered or plate-like mineral, and the optional filler or compoundingingredient.

When the layered or plate-like mineral and a filler such as carbon blackare compounded in a larger amount in total, for example, exceeding 100parts by weight in total, it is preferred to first knead the rubbercomponent, the layered or plate-like mineral, a filler such as carbonblack and another compounding ingredient except for the vulcanizationagent sufficiently at a high temperature, and then to further knead at alow temperature after adding the vulcanization agent. The kneading at ahigh temperature may be carried out at two or more stages, if desired.

When the layered or plate-like mineral and a filler such as carbon blackare compounded in a smaller amount in total, for example, 100 parts byweight or less in total, the electric power to be consumed can bereduced to enhance the productivity by employing a step (a) forpre-kneading the rubber component. In the subsequent kneading step (b)for kneading the pre-kneaded rubber component, the layered or plate-likemineral and the other compounding ingredients, it is preferred to addall the compounding ingredients simultaneously and then carry out thekneading by single stage, because the productivity can be furtherenhanced.

In the pre-kneading step (a), only the rubber component is masticated ina kneader such as Burberry mixer. The mastication is preferably carriedout for 10 sec or more in the present invention. By masticating for 10sec or more, a possible agglomeration of the layered or plate-likemineral on the surface of a rotor can be prevented in the subsequentkneading step, thereby ensuring a good air permeation resistance andflexural fatigue resistance of the resultant vulcanized rubbercomposition. Since a long mastication is likely to reduce theproductivity, it is more preferred to carry out the mastication for 10to 60 sec. If the kneading is carried out by single stage while omittingthe pre-kneading step, the layered or plate-like inorganic filler islikely to agglomerate on the surface of the rotor, thereby failing to,in some case, obtain a sufficient air permeation resistance and flexuralfatigue resistance of the resultant vulcanized rubber composition.

In the kneading step (b), the masticated rubber component is kneadedwith the layered or plate-like inorganic filler, carbon black and theother compounding ingredients. If this kneading step is carried out bysingle stage, the kneading time is preferred to be from one to fourminutes. When the kneading time is less than one minute, the filler islikely to be dispersed insufficiently. When the kneading is carried outover four minute, the vulcanization begins partially during the kneadingto cause the reduction of the air permeation resistances and flexuralfatigue resistance of the resultant vulcanized rubber composition.

When the kneading step is carried out by single stage, it is preferredto control the temperature of the rubber composition at the end of thekneading to 130° C. or lower. If exceeding 130° C., the rubbercomposition is likely to be vulcanized during the kneading, thereby theair permeation resistance and flexural fatigue resistance of theresultant vulcanized rubber composition are deteriorated.

Since the fillers are well dispersed by following the above productionmethod of the present invention, a rubber composition excellent in airpermeation resistance, flexural fatigue resistance and low-temperaturedurability can be produced with good productivity.

The type of the kneading machine is not specifically limited, and may besuitably selected from those employed in the rubber art, such as, aclosed mixer such as Banbury mixer and intermix, and a roll mixer, withthe closed mixer being preferred.

The rubber composition of the present invention prepared by the abovemethod is suitably used as an inner liner rubber composition for tires.The rubber composition after vulcanization has a dynamic elastic modulusof preferably 800 MPa or less, more preferably 600 MPa at −20° C. undera strain amplitude of 0.1% or less.

The pneumatic tire of the present invention is produced by a knownmethod while forming the inner liner by the above rubber composition.Namely, the rubber composition of the present invention optionallycompounded with the additive mentioned above is extruded, shaped andworked into an inner liner member before subjected to vulcanization. Byforming an inner liner from the rubber composition of the presentinvention, the thickness of the inner liner can be reduced, therebymaking it easy to produce a tire having an inner liner of a small gauge.

FIG. 2 is a partial cross-sectional, schematic view showing a tirehaving an inner liner made of the rubber composition of the presentinvention. Particles 12 of the layered or plate-like mineral dispersedin an inner liner layer 11 are orientated so that the surface thereofcrosses the thickness direction of the inner liner layer, i.e., thesurface thereof is parallel or nearly parallel to the surface of theinner liner layer. The arrow shows the flow pass of air from the insideof tire to a carcass layer when air leakage occurs. As shown in thefigure, air entered into the inner liner is prevented from goingstraight forward by the layered or plate-like mineral and forced to goaround the layered or plate-like mineral, thereby taking a long way forpassing through the inner liner.

Thus, a low air permeation of the inner liner made of the rubbercomposition of the present invention can be presumed to be attained bythe prevention of air inside a tire from passing through the inner linerbecause of the orientation of the particles of the layered or plate-likemineral compounded with the rubber composition in the same direction. Incontrast, a clay having a large aspect ratio, which has beenconventionally compounded into a inner liner rubber, is difficult to beuniformly dispersed into a rubber by the kneading process and forms anagglomerate. The agglomerate acts as a fracture point in a vulcanizedrubber composition to reduce the flexural resistance and thelow-temperature durability of an inner liner, thereby deteriorating thedurability of a tire. By the use of the layered or plate-like mineralhaving a limited aspect ratio of 3 or more and less than 30, the presentinvention succeeds to reduce the air permeability of the inner linerwithout reducing the flexural resistance and the low-temperaturedurability. In addition, by simultaneously using carbon black as areinforcing filler while regulating each compounding amount and thetotal compounding amount within the specific ranges, the effect of thelayered or plate-like mineral can be further enhanced.

FIG. 3 is a partial cross-sectional view showing a pneumatic tire of thepresent invention, in which the tire has a carcass layer 2 comprising acarcass ply which extends around a bead core 1 and has radiallyextending cords, an inner liner 3 disposed radially inward of thecarcass layer, a belt portion comprising two belt plies 4 disposedradially outward of a crown portion of the carcass layer, a treadportion 5 disposed radially outward of the belt portion, and a sidewallportion 6 disposed at right and left lateral sides of the tread portion.The inner liner 3 of the pneumatic tire having the above structure ismade of the rubber composition of the present invention. The thickness Dof the inner liner 3 may be varied depending on tire size, and generally0.2 to 2.5 mm. Preferably, 0.2 to 1.2 mm for a tire of passengervehicle, 0.8 to 2.5 mm for a tire of truck and bus, and 1 to 2 mm for atire of airplane. The gas for inflating the tire may be air or nitrogen.

The present invention will be described in more detail with reference tothe following examples. However, it should be noted that the followingexamples are merely illustrative and not intended to limit the scope ofthe present invention thereto.

In Examples 1 to 28 and Comparative Examples 1 to 6, the properties weremeasured by the following methods.

(1) Air Permeation Resistance of a Vulcanized Rubber Composition

According to the method A (pressure difference method) of JIS K7126-1987“Test of Gas Permeability of Plastic Film and Sheet”, the airpermeability constant of each specimen was measured. In Tables 1 to 3,the reciprocal number of the air permeability constant is shown by theindex number taking the value of Comparative Examples 1, 3 or 5 as 100.The larger the index number, the better the air permeation resistance.

(2) Flexural Fatigue Resistance of a Vulcanized Rubber Composition

According to de Mattia test of JIS K6260-1999, the number of therepeated flex cycle until the specimen was broken was measured at roomtemperature under 40 mm stroke. The results are shown in Tables 1 to 3by the index number taking the result of Comparative Example 1 or 5 as100. The larger the index number, the better the flexural fatigueresistance.

(3) Low-Temperature Durability of a Vulcanized Rubber Composition

According to the low-temperature impact brittleness test of JISK6301-1995, the brittle temperature was measured. The results are shownin Table 2 by the index number taking the result of Comparative Example3 as 100. The larger the index number, the better the low-temperaturedurability.

(4) Dynamic Storage Modulus (−20° C.) of a Vulcanized Rubber Composition

The dynamic storage modulus of a specimen of 2.0 mm thick, 5.0 mm wideand 20 mm long was measured at −20° C. using a spectrometer manufacturedby Toyo Seiki Seisakysho Co., Ltd. under conditions of a static initialload of 150 g, an average strain amplitude of 0.1% and a frequency of 32Hz. The results are shown in Table 3.

(5) Modulus of Non-Vulcanized Rubber Composition

The tensile stress at 50% elongation M₅₀ of a JIS No. 5 specimen (ringspecimen) was measured according to JIS K6301-1995 at a tensile speed of100±5 mm/min. The results are shown in Tables 1 and 2 by the indexnumbers taking the result of Comparative Example 1 or 3 as 100. Thelarger the index number, the larger the modulus.

EXAMPLES 1 TO 13 AND COMPARATIVE EXAMPLES 1 AND 2

The compounding ingredients of respective amounts shown in Table 1, 5.0parts by weight of a spindle oil, 1.0 part by weight of zinc white, 0.5part by weight of a vulcanization accelerator (Nocceler NS (trade mark)manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.;N-tert-butyl-2-benzothiazyl-sulfenamide), and 1.0 part by weight ofsulfur were kneaded in a Banbury mixer by two-stage process of ahigh-temperature kneading and a low-temperature kneading. The resultantnon-vulcanized rubber composition was measured on its modulus. Then, thenon-vulcanized rubber composition was vulcanized at 180° C. for 10 min,and the vulcanized rubber composition was evaluated on the airpermeation resistance at 60° C. and the flexural fatigue resistance. Theresults are shown in Table 1.

TABLE 1 Comparative Examples Examples 1 2 1 2 3 Formulation (part byweight) Natural rubber RSS^(#)1 — — 55 55 — 1,2BR*¹ — — — — —Chlorinated butyl rubber*² 100 100 45 45 100 Modified butyl rubber*³ — —— — — FRR*⁴ — — — — — Nylon 6 short fiber*⁵ — — — — — Carbon black GPF55 10 10 20 20 Carbon black SAF — — — — — Crown clay*⁶ — 200 — — — Flatclay*⁷ — — 300 200 150 Coupling agent Si69*⁸ — — — — — Dispersionimprover*⁹ — 3 5 3 2 Resorcinol — — — — — Hexamethylenetetramine — — — —— Results (by index number) Air permeation resistance (60° C.) 100 150330 300 450 Flexural fatigue resistance 100 90 90 110 150 Modulus 100110 190 180 150 Examples 4 5 6 7 8 Formulation (part by weight) Naturalrubber RSS^(#)1 — — — — — 1,2BR*¹ — — — 20 — Chlorinated butyl rubber*²100 94 100 80 100 Modified butyl rubber*² — — — — — FRR*⁴ — 4.5 — — —Nylon 6 short fiber*⁵ — — — — — Carbon black GPF 20 20 — 20 10 Carbonblack SAF — — 20 — — Crown clay*⁶ — — — — — Flat clay*⁷ 200 150 150 130100 Coupling agent Si69*⁸ — — — 2 1 Dispersion improver*⁹ 3 2 2 — 1Resorcinol — — — — — Hexamethylenetetramine — — — — — Results (by indexnumber) Air permeation resistance (60° C.) 650 430 450 360 440 Flexuralfatigue resistance 100 135 145 100 140 Modulus 170 220 160 170 190Examples 9 10 11 12 13 Formulation (part by weight) Natural rubberRSS^(#)1 — 20 — — — 1,2BR*¹ — — — — — Chlorinated butyl rubber*² 100 80100 100 Modified butyl rubber*³ — — — — 86 FRR*⁴ — — — — 21 Nylon 6short fiber*⁵ — — — 3 — Carbon black GPF — 10 35 20 20 Carbon black SAF— — — — — Crown clay*⁶ — — — — — Flat clay*⁷ 100 100 150 130 150Coupling agent Si69*⁸ 1 — — — — Dispersion improver*⁹ 1 — 2 1 2Resorcinol — — — 2 — Hexamethylenetetramine — — — 1.3 — Results (byindex number) Air permeation resistance (60° C.) 100 150 330 300 450Flexural fatigue resistance 100 90 90 110 150 Modulus 100 110 190 180150 Note *¹Syndiotactic Poly(1,2-butadiene) (manufactured by JSRCorporation; JSR RB810, trade mark) *²Enjay Butyl HT10-66 (trade mark;manufactured by Enjay Chemical Co., Ltd.) *³Halogenatedisobutylene-p-methylstyrene copolymer (manufactured by Exxon; EXPRO50,trade mark) *⁴FRR (manufactured by Ube Industries, Ltd.; HE 0100, trademark, natural rubber:nylon short fiber = 2:1 (by weight)) *⁵Nylon 6short fiber (manufactured by Unitika, Ltd.; average diameter = 3.3 dtex,average length = 1 mm) *⁶Crown clay (manufactured by Shiraishi CalciumCo., Ltd.; Crown Clay-S, trade mark) *⁷Flat clay (manufactured by J. M.Huber Co., Ltd.; POLYFIL DL, trade mark; aspect ratio = 10) Flat clay isa kaolinic clay with a larger aspect ratio. *⁸Si69 (trade mark,manufactured by Degussa Aktiengesellschaft) *⁹Dispersion improver(dimethylstearylamine; manufactured by Kao Corporation, DM80, trademark)

As compared with a conventional rubber composition for inner liner usedin Comparative Example 1, each rubber composition of Examples 1 to 13was largely improved in its air permeation resistance while retaining,at least, the flexural fatigue resistance, and simultaneously largelyincreased in the modulus in the non-vulcanized state.

EXAMPLES 14 TO 21 AND COMPARITIVE EXAMPLES 3 AND 4

The compounding ingredients of respective amounts shown in Table 2, 10parts by weight of a spindle oil, 1.5 parts by weight of zinc white, 0.5part by weight of a vulcanization accelerator (Nocceler NS (trade mark)manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.;N-tert-butyl-2-benzothiazylsulfenamide), and 1.0 part by weight ofsulfur were kneaded in a Burberry mixer by two-stage process of ahigh-temperature kneading and a low-temperature kneading. The resultantnon-vulcanized rubber composition was evaluated on its modulus. Then,the non-vulcanized rubber composition was vulcanized at 180° C. for 10min, and the resultant vulcanized rubber composition was evaluated onthe air permeation resistance at 60° C. and the flexural fatigueresistance. The results are shown in Table 2.

TABLE 2 Comparative Examples Examples 3 4 14 15 16 Formulation (part byweight) Natural rubber RSS^(#)1 50 70 70 80 70 1,2BR*¹⁰ — — — — —Brominated butyl rubber*¹¹ 50 30 30 20 30 Modified butyl rubber*³ — — —— — FRR*⁴ — — — — — Nylon 6 short fiber*⁵ — — — — — Carbon black GPF 5555 20 20 20 Carbon black SAF — — — — — Crown clay*⁶ — — — — — Flatclay*⁷ — — 80 80 100  Coupling agent Si69*⁸ — — — — — Dispersionimprover*⁹ — — — —  2 Resorcinol — — — — — Hexamethylenetetramine — — —— — Results (by index number) Air permeation resistance (60° C.) 100  70130  105  220  Flexural fatigue resistance 100  110  110  118  110 Modulus 100  110  110  115  130  Examples 17 18 19 20 21 Formulation(part by weight) Natural rubber RSS^(#)1 70 70 50 70 64 1,2BR*¹⁰ — — 20— — Chlorinated butyl rubber*² 30 — 30 30 30 Modified butyl rubber*³ —30 — — — FRR*⁴ — — — — 4.5 Nylon 6 short fiber*⁵ — — — 3 — Carbon blackGPF — — 20 20 20 Carbon black SAF 20 — — — — Crown clay*⁶ — — — — — Flatclay*⁷ 100 200 100 100 100 Coupling agent Si69*⁸ — — — — — Dispersionimprover*⁹ 2 3 2 2 2 Resorcinol — — — 2 — Hexamethylenetetramine — — —1.3 — Results (by index number) Air permeation resistance (60° C.) 215330 230 250 250 Flexural fatigue resistance 110 110 115 110 110 Modulus170 270 170 500 480 Note *¹⁰Syndiotactic poly(1,2-butadiene)(manufactured by JSR Corporation; RB100, trade mark) *¹¹Bromobutyl 2244(trade mark; manufactured by JSR Corporation) *² to *⁹ as defined inTable 1.

The following would appear from the results of Table 2. The airpermeation resistance and the low-temperature brittle resistance(low-temperature durability) of the vulcanized rubber composition andthe modulus of the non-vulcanized rubber composition are allinsufficient when the rubber composition does not contain the layered orplate-like mineral but contains 40 parts by weight or more of thebutyl-based rubber as in Comparative Example 3. When the rubbercomposition does not contain the layered or plate-like mineral butcontains 40 parts by weight or less of the butyl-based rubber as inComparative Example 4, the low-temperature brittle resistance and themodulus of the non-vulcanized rubber composition are relatively good ascompared with Comparative Example 3, but the air permeation resistanceis poor. In contrast, in the examples of the present invention, the airpermeation resistance and the low-temperature brittle resistance of thevulcanized rubber and the modulus of the non-vulcanized rubber areimproved and well-balanced.

EXAMPLES 22 TO 28 AND COMPARATIVE EXAMPLES 5 AND 6

The compounding ingredients of respective amounts shown in Table 3, 2.0parts by weight of zinc white, 1.0 part by weight of a vulcanizationaccelerator DM (dibenzothiazyldisulfide) and 1.0 part by weight ofsulfur were kneaded in a Banbury mixer by two-stage process of ahigh-temperature kneading and a low-temperature kneading, therebypreparing each non-vulcanized rubber composition.

The non-vulcanized rubber composition was vulcanized at 180° C. for 10min, and each resultant specimen was evaluated on the air permeationresistance at 60° C., the flexural fatigue resistance and the dynamicstorage modulus at −20° C. The results are shown in Table 3.

TABLE 3 Comparative Examples Examples 5 6 22 23 Formulation (part byweight) Natural rubber RSS^(#)1 — — — — Brominated butyl rubber*¹¹ 100100 100 100 Carbon black GPF 70 10 10 10 Crown clay*⁶ — 60 — — Flatclay*⁷ — — 60 100 Dispersion improver*⁹ — — 1.0 1.0 Results Airpermeation resistance (60° C.) 100 100 300 390 (by index number)Flexural fatigue resistance 100 80 200 155 (by index number) dynamicstorage modulus (−20° C.) 540 450 460 550 (MPa) Examples 24 25 26 27 28Formulation (part by weight) Natural rubber RSS^(#)1 — — 15 — —Brominated butyl rubber*¹¹ 100 100 85 100 100 Carbon black GPF 10 50 1010 10 Crown clay*⁶ — — — — — Flat clay*⁷ 170 60 60 60 170 Dispersionimprover*⁹ 1.0 1.0 1.0 — 3.0 Results Air permeation resistance 430 380110 300 430 (60° C.) (by index number) Flexural fatigue resistance 100160 260 150 330 (by index number) dynamic storage modulus 665 605 160460 660 (−20° C.) (MPa) Note *⁶, *⁷ and *⁹ as defined in Table 1. *¹¹ asdefined in Table 2.

As seen from Table 3, Examples 22 to 28 are superior to ComparativeExamples with respect to the balance of the air permeation resistance,the flexural fatigue resistance and the dynamic storage modulus of thevulcanized rubber compositions.

In the following Examples 29 to 43 and Comparative Examples 7 to 13theproperties were measured by the following methods.

(1) Air Permeation Resistance of a Vulcanized Rubber Composition

According to the method A of JIS K7126-1987, the air permeability wasmeasured using an air permeability machine. The results are shown inTables 4 to 6 by index numbers taking the air permeability ofComparative Example 7, 10 or 12 as 100. The smaller the index number,the lower the air permeability.

(2) Flexural Fatigue Resistance of a Vulcanized Rubber Composition

According to the flex test method of JIS K6260-1999, each test piece ofvulcanized rubber was prepared, which was subjected to flex test tomeasure the time required until a crack of 10 mm long occurred on thetest piece. The results are shown in Tables 4 to 6 by index numberstaking the measured time of Comparative Example 7, 10 or 12 as 100. Thelarger the index number, the better the flexural fatigue resistance.

(3) Low-Temperature Durability of a Vulcanized Rubber Composition

According to the low-temperature impact brittleness test of JISK6301-1995, each test piece was prepared and subjected to thelow-temperature impact brittleness test to measure the impact brittletemperature. The difference (° C.) between the measured impact brittletemperature and that of Comparative Example 10 or 12 is shown in Tables5 and 6. The smaller the difference, the better the low-temperaturedurability.

(4) Strength of a Non-Vulcanized Rubber Composition

A non-vulcanized rubber composition was sufficiently warmed by 8-inchrolls to prepare a sheet of 4 mm thick, which was then cut into a testpiece by a JIS No. 5 cutting die. The test piece was subjected to astrength test according to JIS K6251-1993 to measure the breakingstrength. The results are shown in Table 4 by the index numbers takingthe breaking strength of Comparative Example 7 as 100. The larger theindex number, the higher the strength of the non-vulcanized rubbercomposition.

(5) Agglomerate Content of Non-Vulcanized Rubber Composition

The particle size distribution of particles having a maximum particlesize of 20 μm or less was measured using Dispergrader 1000 (manufacturedby Optigrade Co., Ltd.). By comparing the photographic image of eachtest piece and a reference photograph, the results were ranked takingthe result of Comparative Example 12 as 100 (Table 6). The agglomeratecontent expresses the degree of dispersion of fillers. The larger therank, the better the dispersion.

EXAMPLES 29 TO 31 AND COMPARATIVE EXAMPLES 7 TO 9

The compounding ingredients of respective amounts shown in Table 4, 10parts by weight of a process oil, 3.0 parts by weight of zinc white, 2.0parts by weight of stearic acid, and 1.0 part by weight of sulfur werekneaded in a Banbury mixer by two-stage process of a high-temperaturekneading and a low-temperature kneading. The resultant non-vulcanizedrubber composition was evaluated on its strength. Then, thenon-vulcanized rubber composition was vulcanized at 145° C. for 45 min,and each test piece of the resultant vulcanized rubber composition wasevaluated on the air permeation resistance and the flexural fatigueresistance. The results are shown in Table 4.

TABLE 4 Comparative Examples 7 8 9 Formulation (part by weight) Naturalrubber RSS^(#)1 — — 15 Brominated butyl rubber*¹¹ 100 100 85 Carbonblack A*¹² 50 — 50 Carbon black B*¹³ — 80 — Clay*¹⁴ — — — Results Airpermeation resistance 100 85 150 (by index number) Flexural fatigueresistance 100 100 120 (by index number) Strength of non-vulcanized 10080 150 rubber (by index number) Comparative Examples 29 30 31 32Formulation (part by weight) Natural rubber RSS^(#)1 — — 15 — Brominatedbutyl rubber*¹¹ 100 100 85 100 Carbon black A*¹² 30 20 20 50 Carbonblack B*¹³ — — — — Clay*¹⁴ 40 80 80 30 Results Air permeation resistance67 50 85 66 (by index number) Flexural fatigue resistance 390 280 400125 (by index number) Strength of non-vulcanized 100 105 180 130 rubber(by index number) Note *¹²Carbon black A: DBP 85 m/L/100 g, IA 35 mg/g.*¹³Carbon black B: DBP 45 mL/100 g, IA 16 mg/g. *¹⁴Clay: kaolinic clayhaving an aspect ratio of 12.5. *¹¹ as defined in Table 2

The results of Table 4 show that the rubber compositions of the presentinvention are excellent in the air permeation resistance and flexuralfatigue resistance after vulcanization, and excellent in the strengthand workability before vulcanization.

EXAMPLES 33 TO 36 AND COMPARATIVE EXAMPLE 10 AND 11

The compounding ingredients shown in Table 5 were kneaded in a Banburymixer by two-stage process of a high-temperature kneading and alow-temperature kneading, thereby preparing each non-vulcanized rubbercomposition for an inner liner. The non-vulcanized rubber compositionwas vulcanized at 145° C. for 45 min, and each test piece of theresultant vulcanized rubber component was evaluated on the airpermeation resistance, the flexural fatigue resistance and thelow-temperature durability. The results are shown in Table 5.

TABLE 5 Comparative Examples 10 11 Formulation (part by weight)Brominated butyl rubber*¹¹ 100 100 Carbon black*¹⁵ 60 50 Clay (aspectratio 12.5)*¹⁶ — — Clay (aspect ratio 38.0)*¹⁷ — 30 Process oil 10 10Zinc white 3 3 Stearic acid 2 2 Sulfur 1 1 Results Air permeationresistance 100 69 (by index number) Flexural fatigue resistance 100 80(by index number) Low-temperature durability 0 0 (temperaturedifference) Examples 33 34 35 36 Formulation (part by weight) Brominatedbutyl rubber*¹¹ 100 100 100 100 Carbon black*¹⁵ 50 40 30 40 Clay (aspectratio: 12.5)*¹⁶ 30 20 40 30 Clay (aspect ratio: 38.0)*¹⁷ — — — — Processoil 10 10 10 10 Zinc white 3 3 3 3 Stearic acid 2 2 2 2 Sulfur 1 1 1 1Results Air permeation resistance 66 81 59 70 (by index number) Flexuralfatigue resistance 125 355 390 255 (by index nuber) Low-temperaturedurability −0.5 −2.5 −1 −1 (temperature difference) Note *¹⁵N660(manufactured by Cabot Corporation) *¹⁶Polyfil DL (trade mark,nanufactured by J.M. Huber Corporation) *¹⁷Clay having a high aspectratio prepared by sieving commercially available clay.

As seen from Table 5, the rubber compositions for an inner liner of thepresent invention are excellent in the air permeation resistance,flexural fatigue resistance and low-temperature durability aftervulcanization.

EXAMPLES 37 TO 43 AND COMPARATIVE EXAMPLES 12 AND 13

A rubber component (100 parts by weight) shown in Table 6, a filler orfillers of respective amount(s) shown in Table 6, 10 parts by weight ofa process oil (spindle No. 2 manufactured by Japan Oil Co., Ltd.;ko-grade blown asphalt manufactured by Japan Oil Co., Ltd.), 3 parts byweight of zinc white, 1 part by weight of a vulcanization accelerator(Nocceler DM-P, trade mark, manufactured by Ouchi Shinko ChemicalIndustrial Co., Ltd.), and 1 part by weight of sulfur were compounded inthe following manner to prepare a non-vulcanized rubber composition.

The rubber component shown in Table 6 was masticated in a Banbury mixerfor a period of time shown in Table 6 (step (a)). Then, after adding thefiller or fillers and the other compounding ingredients shown in Table6, the kneading was carried out by single stage for a period of timeshown in Table 6 while controlling the kneading so as to reach thetemperature (dump temperature) shown in Table 6 at the end of kneading(step (b)). With such a kneading method, the electric power consumptionwas reduced much more as compared with the preparation of a rubbercomposition of the same compounding formulation by two-stage kneading ata high temperature and a low temperature.

The non-vulcanized rubber composition was evaluated on its agglomeratecontent. Then, the non-vulcanized rubber composition was vulcanized at145° C. for 45 min, and the test piece of each vulcanized rubbercomposition was evaluated on the air permeation resistance, flexuralfatigue resistance and low-temperature durability. The results are shownin Table 6.

TABLE 6 Comparative Examples Examples 12 13 37 38 39 Rubber component(part by weight) Natural rubber RSS^(#)1 10 10 10 30 10 Brominated butylrubber*¹¹ 90 90 90 70 90 Filler (part by weight) Carbon black*¹⁸ 60 5050 30 40 Crown clay*⁶ — 20 — — — Clay II*²⁰ (aspect ratio: 15) — — — 4030 Clay III*²¹ (aspect ratio: 40) — — 20 — — Step (a) Pre-kneading time(sec) 20 20 20 20 20 Step (b) Kneading time (sec) 180 180 180 180 200Dump temperature (° C.)*²² 120 120 120 115 120 Properties of rubbercomposition Agglomerate content (rank) 100 90 90 155 140 Air permeationresistance 100 95 89 59 70 (by index number) Flexural fatigue resistance100 90 70 370 240 (by index number) Low-temperature durability 0 0.8 2−1 −0.5 (by index number) Examples 40 41 42 43 Rubber component (part byweight) Natural rubber RSS#1 — 20 20 20 Brominated butyl rubber*¹¹ 10080 80 80 Filler (part by weight) Carbon black*¹⁸ 40 35 40 60 Crownclay*⁶ — — — — Clay II*²⁰ (aspect ratio: 15) 30 35 70 10 Clay III*²¹aspect ratio: 40) — — — — Step (a) Pre-kneading time (sec) 10 30 50 60Step (b) Kneading time (sec) 240 180 180 180 Dump temperature (° C.)*²²126 110 115 130 Properties of rubber composition Agglomerate content(rank) 163 135 159 128 Air permeation resistance 68 72 76 80 (by indexnumber) Flexural fatigue resistance 200 270 140 240 (by index number)Low-temperature durability −0.5 −0.5 −0.5 −0.5 (by index number) Note*⁸Carbon black: N660 (manufactured by Cabot Co., Ltd.) *¹⁹Clay I (crownclay having an aspect ratio of 3) *²⁰Clay II (kaolinic clay having anaspect ratio of 15) *²¹Clay III (kaolinite soft clay having an aspectratio of 40) *²²Dump temperature (temperature of rubber compositionbeing taken out of a Banbury mixer after kneading) *¹¹ as defined inTable 2.

As seen from Table 6, the rubber compositions of Examples 37 to 43 aregood in the dispersion of the filler and excellent in the air permeationresistance, flexural fatigue resistance and low-temperature durabilityas compared with the rubber compositions of Comparative Examples.

Industrial Applicability

As described above in detail, the rubber composition for an inner linerof the present invention is remarkably improved in the air permeationresistance and also improved in the workability as compared withconventional rubber compositions compounded with a butyl rubber.Therefore, the rapture and hole defect of a non-vulcanized sheet duringthe tire construction can be avoided. By using the rubber composition,the gauge of an inner liner for a pneumatic tire can be reduced whilemaintaining the inner pressure of tire, thereby reducing the weight oftire. In addition, the production process of the present inventionprovides a rubber composition compounded with the layered or plate-likemineral in good productivity without detracting the properties.

1. A pneumatic tire, comprising an inner liner made of a rubbercomposition comprising 100 parts by weight of a rubber componentcomprising at least one butyl-based rubber, 10 to 50 parts by weight ofa layered or plate clay having an aspect ratio of from 3 to less than30, and 10 to 60 parts by weight of a GPF carbon black, wherein therubber composition does not comprise a dispersion improver, and thelayered or plate clay is oriented so that its surface crosses athickness direction of the inner liner.
 2. The pneumatic tire accordingto claim 1, wherein the butyl-based rubber comprises a halogenatedrubber.
 3. The pneumatic tire according to claim 1, wherein the layeredor plate clay comprises a kaolinic clay or a sericitic clay.
 4. Thepneumatic tire according to claim 1, wherein a total compounding amountof the clay and the carbon black is 50 parts by weight or more.
 5. Thepneumatic tire according to claim 1, wherein the carbon black has aniodine adsorption of 40 mg/g or less and a dibutyl phthalate adsorptionof 100 ml/100 g or less.
 6. The pneumatic tire according to claim 1,wherein 0.3 to 5 parts by weight of an organic short fiber is furthercompounded per 100 parts by weight of the rubber component.
 7. Thepneumatic tire according to claim 6, wherein the organic short fiber ispolyamide fiber.
 8. A pneumatic tire, comprising an inner liner made ofa rubber composition comprising 100 parts by weight of a rubbercomponent comprising at least one butyl-based rubber, 10 to 50 parts byweight of a layered or plate clay having an aspect ratio of from 3 toless than 30, and 10 to 60 parts by weight of a GPF carbon black,wherein the layered or plate clay is oriented so that its surfacecrosses a thickness direction of the inner liner, and 0.1 to 5 parts byweight of a dispersion improver is further compounded per 100 parts byweight of the rubber component.
 9. The pneumatic tire according to claim1 or 8, wherein a dynamic elastic modulus of the rubber compositionafter vulcanization is 800 MPa or less at −20° C. under a strainamplitude of 0.1% or less.
 10. A process for producing the pneumatictire of claim 1, which comprises a step (a) of masticating the rubbercomponent and a step (b) of kneading the masticated rubber componentwith the layered or plate clay and the other compounding ingredients,wherein the rubber component is masticated in a kneading machine in apre-kneading step, and then a kneading treatment is carried out bysingle stage after adding the layered or plate clay and the othercompounding ingredients, and the layered or plate clay and the GPFcarbon black are compounded in an amount of 100 parts by weight or lessin total.
 11. The process according to claim 10, wherein the rubbercomponent is masticated for 10 sec or more in the pre-kneading step, andthen the kneading treatment is carried out for one to four minutes whilecontrolling a temperature of the rubber composition to 130° C. or lowerat the end of the kneading treatment.